Currently, a Radio Access Network (RAN) of a Long Term Evolution (LTE) network, rather than a RAN of a 2nd-Generation (2G)/3rd-Generation (3G) network, is allowed to access to a core network of a mobile communication system, i.e., a User Equipment (UE) is merely allowed to reside in either the LTE network or the 2G/3G network at the same time. However, along with the development of the communication technology, in future it is probably necessary for the UE to reside in a network through two RANs. For example, it is necessary for a 5th-Generation (5G) network to support the UE to access the network through a plurality of access systems. Hence, there is an urgent need to provide a scheme for allowing the UE to reside in the network through two RANs simultaneously.